441. Arranging Coins
You have a total of n coins that you want to form in a staircase shape, where every k-th row must have exactly k coins.

Given n, find the total number of full staircase rows that can be formed.

n is a non-negative integer and fits within the range of a 32-bit signed integer.

Example 1:
n = 5
The coins can form the following rows:
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Because the 3rd row is incomplete, we return 2.
Example 2:
n = 8
The coins can form the following rows:
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¤ ¤ ¤
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Because the 4th row is incomplete, we return 3.

分析：直接用公式一行代码就能解决

class Solution {
public:
    int arrangeCoins(int n) {
        return (int)((sqrt(8 * (long)n + 1) - 1)/ 2);
    }
};